In ion mobility spectrometry devices, separation of gas-phase ions is accomplished by exploiting variations in ion drift velocities under an applied electric field arising from differences in ion mobility. One well-known type of ion mobility spectrometry device is the High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) cell, also known by the term Differential Ion Mobility Spectrometry (DMS) cell, which separates ions on the basis of a difference in the mobility of an ion at high field strength relative to the mobility of the ion at low field strength. Briefly described, a FAIMS cell comprises a pair of electrodes (e.g., planar electrodes or cylindrical electrodes) separated by a space that defines an ion separation region through which ions are passed. During transit, ions in the separation gap experience an alternating electric field. The alternating field, which can be a bi-sinusoidal RF waveform, has high field portions and low field portions that are asymmetric but of equal area and opposite sign. Ions can have different mobilities in the high and low portions of the cycle. The mobilities of some ions can, for example, be higher during the high period of the waveform compared to the low period of the waveform. Ions of such mobility behavior in FAIMS can deviate away from the inner RF applied electrode and annihilate on the outer electrode over multiple RF cycles of the waveform. However, a DC offset (compensation voltage, or CV) can be applied to the outer electrode to correct for the trajectory such that the ion can be transmitted through the FAIMS device. Alternatively, a time-varying set of ions can be transmitted by sweeping the CV and generating a FAIMS spectrum of ion current versus CV.
FAIMS devices may be used for a variety of purposes. As an MS-compatible separation technique, FAIMS is useful in separating isomers which cannot be resolved by MS or MS/MS alone as isomers have the same mass but different structures. Design of FAIMS devices can involve trading off separation for transmission efficiency or shorter ion transit time, and the high-field and low-field mobilities of isomers often differ from each other, resulting in isomer-specific CV tunings.
While the optimal CV tunings for transmission of different ion species may differ, their CV tuning curves—the relationship between the transmitted flux and the CV for a given ion species—often overlap. This problem is exacerbated as the number of isomers (e.g., in a sample of mixed isomers) to be separated increases, since the likelihood that at least some of the CV tuning curves overlap grows with more isomers. This prevents the use of FAIMS as a mere filter of unwanted components in a mixture of isomers, complicating the use of FAIMS-MS (or FAIMS-MS/MS) for isomer quantification.
FIG. 1 is a simple example of tuning curves for three isomers, used here to highlight the problem presented by overlapping CV tuning curves. The two rightmost tuning curves overlap significantly with the leftmost tuning curve and even more significantly with each other. In an attempt to separate out a single isomer, a CV must be selected for which the other tuning curves exhibit transmission of nearly zero. Considering the isomer corresponding to the middle tuning curve, this is not possible at all. Considering the isomer corresponding to the leftmost tuning curve, at the optimal CV for transmission, the other two isomers are still transmitted at about 5% of the maximum possible intensity. This contribution can be added to the measurement uncertainty if their relative concentrations can be estimated. But if the other isomers are present in excess of the isomer of interest they can be the major contribution to the signal at the detector even at 5% of their peak intensities. Tuning the isomer of interest away from its optimum CV can sometimes remove more of the contributions of the other isomers to the signal, but even when this is possible it comes at the expense of sensitivity.
What is needed is a method of using FAIMS-MS and a corresponding apparatus for quantification of isomers in mixtures having overlapping CV tuning curves.